feat(phase1): add image storage utilities

- Create backend/services/image_storage.py with 4 core functions:
  - sanitize_filename(): remove unsafe chars, limit to 255 chars, convert to lowercase
  - get_unique_filename(): handle collisions with UUID suffix (format: {name}_{uuid8}_{variant}.jpg)
  - ensure_image_directories(): create /images/ root and category subdirs on startup
  - save_image(): save bytes to /images/{category}/{filename}, returns relative path
- Create comprehensive test suite (22 tests) covering all functionality
- Integrate ensure_image_directories() into FastAPI startup event
- Directory structure: /images/{category}/{filename}
- Collision handling: auto-suffix with UUID if filename exists
- All tests passing, pathlib.Path for safe operations

venv/lib/python3.12/site-packages/pyasn1_modules/rfc8358.py

@@ -0,0 +1,50 @@
+#
+# This file is part of pyasn1-modules software.
+#
+# Created by Russ Housley.
+#
+# Copyright (c) 2019, Vigil Security, LLC
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# Digital Signatures on Internet-Draft Documents
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc8358.txt
+#
+
+from pyasn1.type import univ
+
+from pyasn1_modules import rfc5652
+
+
+id_ct = univ.ObjectIdentifier('1.2.840.113549.1.9.16.1')
+
+id_ct_asciiTextWithCRLF = id_ct + (27, )
+
+id_ct_epub = id_ct + (39, )
+
+id_ct_htmlWithCRLF = id_ct + (38, )
+
+id_ct_pdf = id_ct + (29, )
+
+id_ct_postscript = id_ct + (30, )
+
+id_ct_utf8TextWithCRLF = id_ct + (37, )
+
+id_ct_xml = id_ct + (28, )
+
+
+# Map of Content Type OIDs to Content Types is added to the
+# ones that are in rfc5652.py
+
+_cmsContentTypesMapUpdate = {
+    id_ct_asciiTextWithCRLF: univ.OctetString(),
+    id_ct_epub: univ.OctetString(),
+    id_ct_htmlWithCRLF: univ.OctetString(),
+    id_ct_pdf: univ.OctetString(),
+    id_ct_postscript: univ.OctetString(),
+    id_ct_utf8TextWithCRLF: univ.OctetString(),
+    id_ct_xml: univ.OctetString(),
+}
+
+rfc5652.cmsContentTypesMap.update(_cmsContentTypesMapUpdate)